It has long been known that insulated wire can be forced into the gap of a divided terminal without first stripping the insulation. In some cases the terminal edges at the gap are rigid while in others the terminal has stiff yet resilient prongs. The insulation is generally crushed by opposite edges of the terminal as the wire is forced into the gap, to bare areas of the wire for contact. In some instances, there are sharp corners on the terminal at the entrance to the gap. The sharp corners are intended to make short incisions in the insulation extending parallel to the length of the wire, thus facilitating further rupture of the insulation where the conductor of the insulated wire is to make contact with the terminal. Sharpness at those corners may rupture protective oxide-inhibiting plating on the metal conductor and, in the case of stranded conductor wire, it may shear some of the strands.
Special tools are sometimes used to forcibly assemble the wire to the terminal. In other cases, the terminal structure includes a removable cover which is formed to serve as a driver. In general, more-or-less brute force of the terminal acting against the wire driven against it is relied on to crush and part the area of insulation that must be removed in making wire-to-terminal contact. Where stranded wire is used, the crushing action often drives some insulation between the strands, making the prongs bear against insulation, creating unreliable connection. The construction often imposes critical parameters on the design and manufacture of the terminals. Thus, a terminal having a slot bounded by rigid sides or excessively stiff prongs may well be very effective in tearing through wire insulation, but it may fail to make dependable long-term contact with wire's conductors or it may slice conductor strands, depending on the wire size. In a rigid structure, a wire which is disturbed after insertion, as by handling, may loosen and provide intermittent contact. Excessively supple resilient prongs of a terminal may not be consistently effective in stripping insulation as intended.
A widely known form of insulation-rupturing wire terminal involves a strip of metal having an end portion divided lengthwise into a pair of prongs. That terminal characteristically includes three zones: (1) an end zone having a wire-receiving gap; (2) an intermediate zone where edges of the prongs are pre-biased against each other; and (3) an elongated slot with separated edges, terminating where the prongs join the rest of the terminal strip. The slot evidently was considered a manufacturing requirement, and because it adds length to the prongs, the prongs have been stiffly pre-biased toward each other to meet the basic insulation-crushing and wire-contacting functions.
It is known from my U.S. Pat. No. 3,609,642 issued Sept. 28, 1971, that certain materials, especially certain grades of polymeric materials, can be used to cut through polymeric insulation without risking incision into the copper conductor of the insulated wire. Evidently, that principle has not been put to use in wire-stripping connectors.